The preparation of surface relief structures in photosensitive materials is well known. Over the years, the art has learned how to reduce the dimensions of the relief lines or holes (viewed normal to the surface) to the point where they are measured in terms of several microns. This is generally known as micro-fabrication.
In particular, extensive studies have been made of periodic one-dimensional structures prepared in positive type photosensitive materials or photoresists. The photoresist contains a photosensitive polymer which when exposed to light becomes soluble to an appropriate water-based developer. After exposure and development, the initially flat surface of the photoresist becomes a surface relief structure whose depth depends on how much photoresist has been etched away by the developer. When exposed to an intensity variation that is periodic, such as an interference pattern, a periodic surface profile will be formed which is everywhere proportional to the initial interference intensity pattern. Precise relief structures of this type can be made easily over rather large areas using laser interference techniques.
The formation of relief diffraction gratings using laser interference techniques in general involve the exposure of the photoresist to two coherent interfering laser beams. When such beams interfere, there is produced a stationary periodic fringe pattern consisting of maxima and minima of intensity. The spacing between adjacent maxima (or minima) depends upon the optical system used and substantially any spacing can be obtained down to about half the wavelength of the exposing light.
Under usual conditions, the laser is operated under conditions such that there is an approximately Gaussian distribution of intensity across the beam. In other words, the intensity of the laser beam is at a maximum at the center line thereof and is symmetric around the center line and drops off in a radial direction away from the center. In many procedures where photosensitive materials are exposed to laser light, for example, in the formation of holographic gratings by interfering laser beams in photoresist, such a variation of intensity is undesirable because a uniform exposure is required in order to lead to a uniform etch rate in the photoresist. With a Gaussian distribution, the etch rate, and thus the grating groove depth, is always larger at the center than at the edge.
To convert the Gaussian distribution to a more uniform distribution, methods such as using only a small center portion of the laser beam or intercepting the laser beam with an amplitude filter have been employed. However, these methods can cause a significant loss in laser power in the optical system. The art describes a number of these techniques.
Rhodes and Shealy, Refractive Optical Systems for Irradiance Distribution of Collimated Radiation: Their Design and Analysis, Applied Optics, Vol. 19, No. 20, pages 3545-3553, October, 1980 describes a plano-aspherical lens system designed to convert the radially symmetric energy distribution of a beam of collimated light into a collimated beam with a constant or uniform intensity distribution.
Quintanilla and deFrutos, Holographic Filter that Transforms a Gaussian Into a Uniform Beam, Applied Optics, Vol. 20, No. 5, pages 879-880, March, 1981, describes a volume and phase hologram to provide a coherent light beam of enhanced profile uniformity. They also state that a filter of appropriate density could be constructed by photographing a laser beam, but that it would be necessary to maintain a gamma of unity for the photographic process, and that this is difficult.
Lee, Method for Converting a Gaussian Laser Beam into a Uniform Beam, Optics Communications, Vol. 36, No. 6, pages 469-471, March 1981, described an iterative method to provide a phase filter to convert a Gaussian beam into a beam with a more uniform irradiance distribution.
It is accordingly the object of this invention to provide a method and means by which the Gaussian distribution of the laser beam is compensated--for such that the exposure of, e.g., a photoresist to the laser beam in a given area is substantially uniform.